Electrically powered low-lift pallet trucks are common in the warehousing industry. These trucks are designed to travel at varying speeds while carrying a load supported by a pallet on a vertically displaceable fork carriage. Driving a pallet truck while the fork carriage is close to the ground may result in damage to the warehouse floor, the fork carriage or pallet caused by inadvertent contact, such as striking or scraping, with the floor. When operating on an uneven or rough floor, the pallet truck or its load may wobble or shift, increasing the potential for scraping between the floor and fork carriage. Further, contact with the floor while traveling may cause the pallet or load to shift and even spill.
To prevent this situation, a number of pallet truck manufacturers limit travel speed of the pallet truck when the fork carriage is below a minimum height. Although designed to prevent a different problem, a number of forklift manufacturers also limit travel speed, though only when the fork carriage is too high and may cause the forklift to tip over. Despite the differences, both applications use the height of the fork carriage to limit truck performance. It is therefore important to have an accurate and reliable means for determining the carriage height of material handling vehicle.
In response, a number of height determining technologies or algorithms have been employed with varying results. One common approach is to use limit switches, proximity sensors, or the like mounted at fixed heights that directly sense the position of the fork carriage. This information is then inputted to a vehicle controller. In one control methodology for a pallet truck, when the fork carriage is sensed to be above a predetermined height, high speed operation is enabled. When the carriage is sensed to be lower than the predetermined height, the maximum allowable travel speed of the truck is limited appropriately.
This type of control methodology works well as long as all of the height sensors or switches are functioning properly. However, mechanical sensors and switches may wear out, become misaligned, suffer physical abuse, have signal wiring become disconnected either intentionally or unintentionally, and generally suffer from various problems known to both designers and users of forklifts and pallet trucks alike. These problems are exacerbated by the continuous use, constant abuse, hostile environments, and limited maintenance that many pallet trucks endure. Furthermore, sensors and switches, especially when multiples ones are used in redundant systems, increase the manufacturing and maintenance costs associated with pallet trucks over their serviceable life.
The present invention addresses these issues.